1. Field of the Invention
The present invention refers to a method for producing integrated circuits which are mechanically flexible and may be provided contiguously on a common flexible carrier substrate, as well as to their further processing.
2. Description of the Related Art
Integrated circuits are increasingly processed as increasingly thinner chips. The reason for this is the demand for assemblies which are as flat as possible, e.g. in portable devices, in order to provide the highest possible functionality in a small volume. The underlying technology for the production of these thin ICs (IC=integrated circuit) is the thinning technique for semiconductor wafers, by which the thickness of ICs can now be reduced as a standard to thicknesses less than 100 μm. The possibility of producing ICs with a residual thickness within the range of 10-20 μm which, in this range, are becoming increasingly flexible mechanically has also been demonstrated by now.
This mechanical flexibility is advantageous in particular for producing flat or flexible electronic systems and their processing on film substrates. A well-known example is the production of the so-called smart labels, wherein a substrate provided with antenna structures is equipped with integrated circuits for RF identification (RF=radio frequency). The IC is directly connected, without packaging, mechanically to the substrate and electrically to the antenna.
For more complex assemblies, very thin ICs have not yet become generally accepted. The main reason for this is that manufacturers are not in a position, in terms of technology, to process non-packaged components, since this entails very strict requirements in terms of accuracy of placement, structural accuracy of the carrier and the interconnection technology. In the case of extremely thin flexible ICs, there is also the fact that the technique for handling these components becomes substantially more complicated. Usually the manufacturers who process electronic circuits further therefore may advantageously use packaged ICs, which may be processed with a uniform assembly method and larger structural dimensions. Common forms of packaging, however, are not able to achieve the two substantial advantages of ultra-thin chips, namely an extremely flat design and mechanical flexibility.
Packaged devices are provided for further processing in different forms of packaging. To this end, packaged devices are loaded into trays, rails, belts or the like and supplied in this form to fitting machines for assembly. A drawback of the known manufacturing methods for flexible integrated circuits is that a direct and effective further processing cannot be achieved, since the known forms of packaging do not permit a continuous presentation due to lack of mechanical stability.
EP 1028436 describes, for example, thin integrated circuits which are manufactured in a flexible package comprised of a substrate film, wiring, thin IC and encapsulation. The method provided there has the drawback that continuous manufacturing and further processing of the flexible components is not possible, since the flexible film packages used there do not have a continuous coating.
EP 1256983 essentially describes, for example, a method for producing flexible integrated circuits by a transfer method from a semiconductor wafer to a polymer layer. The drawback of the method introduced is that no solutions for the structure, bonding, or contacting, and connection technique for integration into systems can be indicated.
DE 19542883 introduces, e.g., a flexible chip package which uses films. The chip itself is inserted into the film package by means of lamination, but is itself thick and rigid, i.e. the resulting package is, again, not flexible.
DE 19845296 describes, e.g., a method for bonding a circuit chip. Isoplanar bonding techniques are used, by means of which particularly flat designs are possible. The bonding is achieved by means of a silk-screen printing or stamp-printing method, these methods being used to apply a metallization to the thin circuit fitted onto a carrier, in order to connect the pads of the circuit chip to a conductive pattern arranged on the main surface of the carrier substrate. The bonding of thin circuits during the production of flat assemblies is described.
In Christians, W. et. Al.: “Embedded ultra-thin chips in Flex, 4th int. Workshop on Flexible Electronic Systems, Nov. 23, 2005, IZM Munich, the authors describe a method for packaging ultra-thin chips. In the case of the chips presented, a total thickness of about 50-60 μm is assumed, the thickness of the ICs ranging between 20 and 30 μm. The process cycle shall be described more in detail with reference to FIG. 7. FIG. 7 shows process steps 71 to 76, which illustrate the manufacturing method. In a first step 71, a first polyimide layer 71b is initially spun on a fixed carrier 71a, which may be made of glass, for example, and is cured. In a second step 72, BCB (BCB=benzocyclobotene) 72a is applied, by means of a dispenser, as an adhesive for fixing a thin chip.
Then, in a following step 73, a thin chip 73a is placed in the humid adhesive bed and the BCB 72a is cured. Then, in a step 74 a second polyimide layer 74a is spun. By means of a laser, openings 75a can then, in a step 75, be bored to the contact pads of the chip 73a. The bonding, expansion of the connections and the creation of the external contacts then occur in a metallization and patterning process, in a step 76, which illustrates the mounting of the contact surfaces 76a. 
Thereupon, the finished film package is then separated from the rigid carrier, and a flexible chip package is thus obtained. A drawback of the described method is that due to the use of the rigid carrier, a manufacturing process with a continuous provision of the carrier is not possible. Another drawback of this technique is the high requirements placed upon the manufacturing of the openings and the contacts, which result from the fact that an expansion of the contact structures does not occur until after manufacturing of the chip contacts. Another drawback resides in the open expansion of the contact structures. In order to avoid short-circuits with the substrate wiring, either an insulating intermediate sheet must be used, or a structural concept is applied wherein only the contacts on the film package are open.